In general terms, a computer network is a collection of end systems interconnected through one or more routers. Generally, the end systems both send data to other end systems on the network and receive data sent by other end systems on the network. When an end system is a sender of data, it is referred to as a source for that data; whereas, when it is a receiver of data, it is referred to as a destination for the data. Typically, end systems act as both sources and destinations depending upon whether they are sending or receiving data. When acting as a source, the end system sends data in the form of messages over a communication link to a router, which is also known as an intermediate system or gateway. Emanating from the router are a number of other communication links, each one representing a connecting path over which messages can flow back and forth to other routers and end systems within the network. Essentially, the router is a switching element which processes messages by transferring the messages arriving over one link onto another link for transmission to an end system or another router.
Each message comprises a sequence of information bits. Typically, however, the messages sent over the network are not sent as a continuous, uninterrupted stream of bits. Rather, they are divided up into smaller blocks of information called packets, which are then transmitted individually. Each packet has a predetermined maximum length. In addition to a data field which contains the data to be transferred, a packet also includes a header field which contains control information such as format, identifiers which indicate what portion of the message is contained in the packet, the source of the packet and the intended destination of the packet. When the packets which together contain a message reach the destination, the destination processes them by assembling their data fields into proper order to reconstruct the full message.
An important design objective in networks is controlling the flow of packets so that they will not be transmitted at a faster rate than they can be processed by the routers through which the packets will pass or by the destinations. Even in the simplest network consisting of two end systems interconnected by a router, the source may flood the destination if it transmits packets faster than they can be processed by the destination. In more complicated networks consisting of many end systems, numerous routers and alternative communication paths between the end systems, the likelihood of problems from excess communication traffic is significantly greater. This becomes especially true as the number of active end systems on the network increases and if communication speeds of the equipment within the network are mismatched. A mismatch may exist if, for example, a router cannot transfer packets as fast as they are being sent to it by the source. A mismatch may also exist between the speed at which the link can transmit packets, namely the link speed, and the rate at which the router can transfer packets. Predictably, as the complexity of the network increases, achieving an acceptable traffic control also becomes more difficult.
On most networks, at least two basic mechanisms are normally used for dealing with excess traffic arriving at a destination. One mechanism involves the use of buffers and the other involves flow control. In buffered systems, both the routers and the end systems are provided with buffer memory to handle overloads. Arriving traffic which exceeds the processing rate of the device is temporarily stored in the buffer memory until the device can process it. Buffers offer a satisfactory solution to excess traffic problems only if the overload is transitory. If the overload persists for too long, the buffers may become full after which the additional packets are rejected or destroyed.
The other mechanism, generally referred to as flow control, deals with the allocation of resources at the destination, such as memory and processing. Generally, in accordance with flow control, the destination sets a limit on the rate at which each source sending data to the destination may transmit that data. The sources and the destinations coordinate the transfer of data by an exchange of messages containing requests and acknowledgements. Before the source starts sending packets, it will send a request to the destination seeking permission to begin transmission. In response to the request, the destination sends a message containing an identification of the number of packets the source may dispatch toward the destination without further authorization. This number is commonly referred to as the window size. The source then proceeds to transmit the authorized number of packets toward the destination and waits for the destination to verify their receipt. After the destination successfully receives a packet, it sends a message back to the source containing an acknowledgement indicating the successful receipt of the packet and, in some cases, authorizing the source to send another packet. In this way, the number of packets on the network traveling from the source toward the destination will never be more than the authorized window size.
Neither of these mechanisms, however, satisfactorily deals with the distribution of traffic within the network. Even with these mechanisms in place, on a busy network it is likely that many sources will simultaneously send traffic over the network to more than one destination. If too much of this traffic converges on a single router in too short a time, the limited buffer capacity of the router will be unable to cope with the volume and the router will reject or destroy the packets. When this happens, the network is said to be congested.
When the network is congested, network performance degrades significantly. The affected sources have to retransmit the lost or rejected packets. Retransmissions, however, necessarily use network resources such as buffer storage, processing time and link bandwidth to handle old traffic thereby leaving fewer resources for handling those portions of the messages still waiting to be transmitted for the first time. When that occurs, network delays increase drastically and network throughput drops. Indeed, since some network resources are being dedicated to handling retransmissions at a time when the network is already experiencing a heavy load, there is a substantial risk of the congestion spreading and locking up the entire network. As a consequence, it takes the network much longer to extricate itself from congestion than to get into it.
A variety of alternative approaches exist for dealing with network congestion. Generally, the approaches fall into two categories. One category involves placing limitations on the amount of traffic which will be permitted on the network at any given time. The other category involves methods of limiting the spread of congestion once it occurs and then extricating the network from its congested state.
An approach which falls under the first category is the isarithmic method. According to this approach, a user can send a packet over the network only if it has a permit. There are, however, only a limited number of available permits to be shared by all end systems on the network. As a result, the number of packets on the network at any one time is also limited. A proper choice of the number of available permits significantly reduces the likelihood of congestion. The price paid for this method of control, however, is substantial. First, this method may yield an inefficient use of network resources. To protect against the possibility of traffic converging on a single router and causing congestion, network traffic must be limited to a level which is significantly below network capacity. Thus, a slow router may impact end systems that do not even have traffic flowing through that router. Secondly, distributing permits becomes a serious problem. While inactive end systems are holding onto permits, other end systems who need them cannot use the available network resources. And third, the method really does not address the distribution of traffic on the network which is the real cause of network congestion.
Another example from the first category involves the preallocation of buffers at the routers. This approach is used on networks which create a virtual circuit through the router to handle communications between two end systems. A virtual circuit is essentially a channel over the network which is dedicated to handling only the communications between the two end systems and which appears as though it is an actual physical circuit. The virtual circuit, however, is not an actual physical circuit connecting the two end systems but rather is a mechanism for transporting messages between them. When the network establishes the virtual circuit between two end systems, routers along the path over which the packets will pass set aside buffers and other router resources to handle only the traffic between the two end systems. By preallocating buffers in this manner, the routers will always have memory available to store arriving packets until they can be forwarded. As with the isarithmic method, a major drawback to this approach is that it is inefficient. Even during periods of inactivity, buffers and other router resources committed to one virtual circuit cannot be used to handle packet transfers associated with communications between other end systems.
The second category of approaches for dealing with network congestion is commonly referred to as congestion control. Congestion control typically involves feedback which signals the onset of congestion and instructs end systems to decrease the rate at which they initiate transmission of packets. Under one approach, the routers send special messages, commonly referred to as "choke packets" or "source quench packets", to the sources, requiring the sources to reduce their traffic on the network. To determine which sources are to receive the choke packets, the router monitors its communication links to detect when their utilization rates rise above a preselected threshold level. When the threshold level is exceeded, the router sends a choke packet back to the sources that generated the packets which are arriving at the router. In response, the sources decrease their output. The most obvious disadvantage of this approach is that it requires adding traffic, in particular, the choke packets, to the network at a time when the network is least able handle the added traffic. A second disadvantage is it penalizes sources which may not be significant contributors to the traffic overload.
Another method of congestion control which has been used is delay sensitive routing. According to this method, the routers maintain tables which indicate the delays associated with the different paths passing through them. As traffic moves through the network, paths are selected by the routers to yield the lowest delays to the intended destinations. To update the delay tables maintained by the routers, the routers periodically measure the delays on the various paths and then communicate the delay information to each other over the network. As with the previous method, delay-sensitive routing requires adding traffic to the network, which may not be desirable. In addition, delays may vary too quickly to provide an effective method for routing. Moreover, any attempt to keep them current results in high overhead due to the large volume of required updating activity and the inter-router communication of delay information.
A third approach to congestion control involves piggybacking the feedback information onto packets which are traveling back in the direction from which the traffic causing the congestion is coming. Unlike the previous two examples, this does not result in additional traffic. However, the drawback to the approach is that the reverse traffic may not be going to the sources which are the cause of or even participants in the congestion on the forward path.